/Engine/Plugins/MovieScene/MovieRenderPipeline/Source/MovieRenderPipelineCore/Public/MovieRenderPipelineDataTypes.h
This function puts X0 in the middle of PadLeft, X3 in the middle of PadRight. And X1 is "reflected" around PadLeft, and X2 is "reflected" around PadRight.
void InitHelper ( int32 PadLeft, int32 SizeCenter, int32 PadRight )